No preheating will be necessary. About a #0 or #1 tip on an acetylene torch will do a fine job of applying silver solder or brass. I'd probably use silver solder to minimize any possibility of extending the crack from overheating the thin sheet metal. It's also possible the leak is from a rust pinhole, which will be obvious after doing a good cleanup to bare metal, inside and out. If that's the case, the weakened area will be much bigger on the inside so the patch material will need to be applied to the inner surface.
Jerry
